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Old 10-23-2013, 12:54 PM   #21
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The navy taught stability was like a marble in a bowl.

If a marble is in a bowl , after being moved it will eventually return to its position at the bottom of the bowl.

Put the marble on an inverted bowl and there is only instability , one move and the marble departs.

On a boat initial stability is how the boat feels when folks step aboard. Stiff is considered good , dockside.

On a boat underway a far less stiff , roll period of 4-6 seconds for our sized boats is fine, too stiff and its a PUKEATHON boat ride.

Run from side to side to get the boat rolling , down to down on same side is roll period.
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Old 10-23-2013, 01:35 PM   #22
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WTF? Better look up metacenter and sit down with some good diagrams. What is metacenter weight anyway?
Rick – For your better understanding... this is “WTF” Metacenter Weight means to me! LOL

By reading the following: You may discern the meaning of my own officiated term “Metacenter Weight”

Metacenter Weight = Placement of physical weight distributions (as well as their elevations) aboard floating object that can affect a tipping-point past the axis-point wherein a floating object can no longer effect return to upright dead center; therefore capsize. “Weight” placements (hi, low, side to side and/or bow to stern) aboard floating object are important factors that determine said floating objects Metacenter. As well, there are many other items to determine an object’s Metacenter.

http://hal.archives-ouvertes.fr/docs/00/39/09/80/PDF/Archimedes_force.pdf

On the buoyancy force and the metacentre
Jacques Mégel and Janis Kliava
UFR de Physique, Université de Bordeaux 1
351 cours de la Libération,
33405 Talence cedex, France

Page 3: Three years later, Euler in Scientia navalis (1749) gave a general criterion of the ship stability, based on the restoring moment: 11 the ship remains stable as far as the couple weight (applied at G) and the buoyancy force (whose line of application passes through C) creates a restoring moment. A change of sign of the latter results in capsizing, and its vanishing in inclined position (at equilibrium it vanishes by definition) corresponds to the overturn angle.

Page 7: For a floating body the location of G depends not only of its shape but also of the distribution of weights inside the hull, which changes with the ship loading. Therefore, we have chosen to focus on the buoyancy force and the related hydrostatic energy without systematically calling to mind the force of gravity and its potential energy.

Page 8: “Interestingly, if we attempt to determine the centre of gravity of a body from vanishing of the corresponding rotational moment in a given position, its vertical coordinate ZG will also remain undefined. Meanwhile, this difficulty is readily overcome by rotating the body through an arbitrary angle about any non-vertical axis. From the viewpoint of such experience, the point of application of the weight can be defined as the “intersection of all vertical lines passing through the centre of gravity for different angular positions of the body”.

Pages 21, 22: VII. Conclusion - We have shown that the location of the point of application of the buoyancy force depends not only on the type of motion of the floating body (translation or rolling/pitching) but, in the latter case, also on the definition of this point. In translation this point remains fixed with respect to the centre of gravity of the body while in rolling/pitching it is subject to a duality. Namely, from the viewpoint of the work-energy relation it is fixed with respect to the centre of buoyancy while from the viewpoint of the rotational moment it is located at the metacentre. This peculiarity of the buoyancy force is due to the fact that, whereas the immersed body can still be considered as a rigid one, the shape of the displaced fluid does not remain constant in the course of the motion. Indeed, in the case of a completely immersed body, the shape of displaced fluid remains fixed, so that metacentre and the centre of buoyancy coincide. The concept of non-uniqueness of the point of application of a resultant force seems quite unusual; nevertheless, as we have shown, this non-uniqueness is an inherent feature of the buoyancy hal-00390980, version 1 - 4 Jun 2009 22 force. While this finding is not expected to bring about changes in practical applications, it has a certain fundamental and educational interest for the mechanics of floating bodies. It would be interesting to find out whether some other physical forces do possess a similar non-uniqueness of the point of application. Using the general approach based on the hydrostatic energy formalism, we have shown that the
various definitions of the metacentre (“geometrical”, “Bouguer’s” and “dynamical”), in fact, concern one and the same distinct point of the immersed body. This finding holds (i) for any shape of the immersed body and (ii) not only in the vicinity of equilibrium but also for any angle of heel. Besides, from the viewpoint of the rotational moment the metacentre proves to be the point of application of the buoyancy force in the rolling/pitching motion. These findings shed new light on the long-standing concept of the metacentre. Another, more practical, implication of this study concerns the criterion of ship stability in relation to the location of the metacentre. Indeed, the metacentric height is proportional to the angular derivative of the lever arm of the buoyancy force. Thus, the character (restoring or overturning) of the rotational moment of the buoyancy force depends on the sign (resp., positive or negative) of the height of the metacentre above the line of flotation, and the absolute value of this height determines the rate of its angular dependence. The model developed in the present study allows one to get analytical expressions for the location of the metacentre for the floating body of an arbitrary shape. Thus, it presents a certain interest for teaching and practice of naval mechanics and engineering, as well.
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Old 10-23-2013, 03:23 PM   #23
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The closest to a self righting boat on this forum may be the Willards and Fales. All have ballasted hulls. The 30 Willards have 2 tons of ballast in the keel.
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Old 10-23-2013, 03:46 PM   #24
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While cruising the Bahamas on a Commodore Ralph Munroe reproduction, I had the chance to read a book the owner had of conversations between Nathanael Herreshoff and Commodore Ralph Munroe, both noted yacht designers over similar subject of ballast and extreme deep draft vs shoal draft and (leeboards for Monroe) for stability. Interesting reading. Captain Nat pointed out how he sailed his boat down to Miami with no issues boasting the stability of his designs. Monroe countered that he too had sailed down from NY in his shoal draft leeboard design, yet Nat had to anchor outside of Biscayne Bay as his draft was too deep to enter, and Munroe was docked at his house in the Grove as he skimmed over the sandbars and shallow water.
Later I read similar discussions between Ted Hood (big proponent of shallow draft centerboard vessels) and other Naval Architects who favored deep draft, heavily ballasted.
Both proponents of the shallow draft, no ballast (Munroe) designs pointed out that the shallower vessels are faster, and can reach places the deep one's can't. South Florida was originally settled and developed due to Monroe designs. Might have been a good thing back then, but in hind sight....
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Old 10-23-2013, 04:55 PM   #25
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While cruising the Bahamas on a Commodore Ralph Munroe reproduction, I had the chance to read a book the owner had of conversations between Nathanael Herreshoff and Commodore Ralph Munroe, both noted yacht designers over similar subject of ballast and extreme deep draft vs shoal draft and (leeboards for Monroe) for stability. Interesting reading. Captain Nat pointed out how he sailed his boat down to Miami with no issues boasting the stability of his designs. Monroe countered that he too had sailed down from NY in his shoal draft leeboard design, yet Nat had to anchor outside of Biscayne Bay as his draft was too deep to enter, and Munroe was docked at his house in the Grove as he skimmed over the sandbars and shallow water.
Later I read similar discussions between Ted Hood (big proponent of shallow draft centerboard vessels) and other Naval Architects who favored deep draft, heavily ballasted.
Both proponents of the shallow draft, no ballast (Munroe) designs pointed out that the shallower vessels are faster, and can reach places the deep one's can't. South Florida was originally settled and developed due to Monroe designs. Might have been a good thing back then, but in hind sight....
Just to clarify, both Commodore Monroe and Ted Hood were proponents of shallow draft and heavy ballast. Which was the only way to achieve adequate righting arms and sail carrying power. Neither leeboards nor centerboards(in the usual sense) have anything to do with stability, they are to counter leeway and provide lift to windward.
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Old 10-23-2013, 05:01 PM   #26
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... Commodore Monroe and Ted Hood were proponents of shallow draft and heavy ballast... Neither leeboards nor centerboards(in the usual sense) have anything to do with stability ...

Yeah but maybe they had a really heavy metacenter?
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Old 10-23-2013, 06:47 PM   #27
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Just to clarify, both Commodore Monroe and Ted Hood were proponents of shallow draft and heavy ballast. Which was the only way to achieve adequate righting arms and sail carrying power. Neither leeboards nor centerboards(in the usual sense) have anything to do with stability, they are to counter leeway and provide lift to windward.
Considering I'm talking the 1870's and 80's- it's safe to assume I meant sailboats and the leeboards were for exactly the purpose they're named. Munroe designs were not "heavily ballasted"- they got their stability from the hull design and what ballast they had was internal. VERY shoal draft. The point I was making was the discussion of displacement VS "getting on top" (planing) has been going on a long time. Each has their merits. Personally though-I like it on top.
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Old 10-24-2013, 02:46 PM   #28
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Considering I'm talking the 1870's and 80's- it's safe to assume I meant sailboats and the leeboards were for exactly the purpose they're named. Munroe designs were not "heavily ballasted"- they got their stability from the hull design and what ballast they had was internal. VERY shoal draft. The point I was making was the discussion of displacement VS "getting on top" (planing) has been going on a long time. Each has their merits. Personally though-I like it on top.
I guess I was confused when you wrote in post #24, "shoal draft and (leeboards for Monroe) for stability." I can find no reference of Monroe using leeboards, could you direct me to one? Perhaps you are thinking of LF Herreshoff's Medowlark, which is a leeboard design?

Again in post #24 you stated, "no ballast (Monroe) designs" This is not the case. Egret, Monroe's 28' flat-bottomed double-ender, carried 75 bricks in her bilge for ballast, which seems a decent load for a 28' boat. Presto, the 41' which was built in New York and sailed to Florida, carried 4.5 tons of iron ballast, at least 1/3rd of her displacement and probably closer to half. I think 40-50% ballast/displacement ratio is heavily ballasted, certainly it's more than no ballast.
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Old 10-24-2013, 03:11 PM   #29
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I guess I was confused when you wrote in post #24, "shoal draft and (leeboards for Monroe) for stability." I can find no reference of Monroe using leeboards, could you direct me to one? Perhaps you are thinking of LF Herreshoff's Medowlark, which is a leeboard design?

Again in post #24 you stated, "no ballast (Monroe) designs" This is not the case. Egret, Monroe's 28' flat-bottomed double-ender, carried 75 bricks in her bilge for ballast, which seems a decent load for a 28' boat. Presto, the 41' which was built in New York and sailed to Florida, carried 4.5 tons of iron ballast, at least 1/3rd of her displacement and probably closer to half. I think 40-50% ballast/displacement ratio is heavily ballasted, certainly it's more than no ballast.
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It's possible the vessel I was cruising on which had the Munroe books onboard was was a reproduction of Medowlark. This was back in 78. Could had sworn the owner said it was a Munroe design. She was named Greta, it was 40', had lee boards, free standing unstayed masts. She was a fixture there in the anchorage of Coconut Grove for decades. The owner took me over to the Barnacle where Munroe lived a 1/4 mile away to see the other designs and the vessel on display. I read the books, saw the designs, and was sold on the shoal vs keel designs, and purchased a 69 Morgan 38' centerboard boat for myself. My point was that the debate between shallow draft planing vessels and deep draft displacement vessels have been going on for many years as documented in the book. Having personally painted the bottom of Greta by heeling her over on Monument Island- between the tides- I certainly remember there was no external ballast, nor does the vessel on display have any external ballast. I know that Herreshoff spent his later years here with Munroe, so maybe his influence rubbed off and hence the Medowlark? One doesn't immediately equate shoal draft leeboard designs to Herreshoff. One thing I DO know is that Munroe didn't build deep draft heavily ballasted boats. His home is still there in the Grove. It's chock a block full of photos, drawings, original furniture, etc.
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